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Storing Tree like Hierarchy Structures With MongoDB

, 16 Jan 2013
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This article demonstrates three approaches for storing tree like structures with NoSQL databases on example of the MongoDB.

Introduction 

In a real life almost any project deals with the tree structures. Different kinds of taxonomies, site structures etc require modeling of hierarchy relations. In this article I will illustrate using first three of five typical approaches of operating with hierarchy data on example of the MongoDB database. Those approaches are:

  • Model Tree Structures with Child References
  • Model Tree Structures with Parent References
  • Model Tree Structures with an Array of Ancestors 
  • Model Tree Structures with Materialized Paths
  • Model Tree Structures with Nested Sets 

Note: article is inspired by another article 'Model Tree Structures in MongoDB' by 10gen, but does not copy it, but provides additional examples on typical operations with tree management. Please refer for 10gen's article to get more solid understanding of the approach.  

Background 

As a demo dataset I use some fake eshop goods taxonomy.

Challenges to address 

In a typical site scenario, we should be able to

  • Operate with tree (insert new node under specific parent, update/remove existing node, move node across the tree)
  • Get path to node (for example, in order to be build the breadcrumb section)
  • Get all node descendants (in order to be able, for example, to select goods from more general category, like 'Cell Phones and Accessories' which should include goods from all subcategories. 

On each of the examples below we:

  • Add new node called 'LG' under electronics
  • Move 'LG' node under Cell_Phones_And_Smartphones node
  • Remove 'LG' node from the tree
  • Get child nodes of Electronics node
  • Get path to 'Nokia' node
  • Get all descendants of the 'Cell_Phones_and_Accessories' node

Please refer to image above for visual representation. 

Tree structure with parent reference 

This is most commonly used approach. For each node we store (ID, ParentReference, Order)

Operating with tree

 Pretty simple, but changing the position of the node within siblings will require additional calculations. You might want to set high numbers like item position * 10^6 for order in order to be able to set new node order as trunc (lower sibling order - higher sibling order)/2 - this will give you enough operations, until you will need to traverse whole the tree and set the order defaults to big numbers again 

 Adding new node

Good points: requires only one insert operation  to introduce the node. 

var existingelemscount = db.categoriesPCO.find({parent:'Electronics'}).count();
var neworder = (existingelemscount+1)*10;
db.categoriesPCO.insert({_id:'LG', parent:'Electronics', someadditionalattr:'test', order:neworder})
//{ "_id" : "LG", "parent" : "Electronics", 
//      "someadditionalattr" : "test", "order" : 40 } 

Updating/moving the node 

Good points: as during insert -  requires only one update operation to amend the node 

existingelemscount = db.categoriesPCO.find({parent:'Cell_Phones_and_Smartphones'}).count();
neworder = (existingelemscount+1)*10;
db.categoriesPCO.update({_id:'LG'},{$set:{parent:'Cell_Phones_and_Smartphones', order:neworder}});
//{ "_id" : "LG", "order" : 60, "parent" : 
//          "Cell_Phones_and_Smartphones", "someadditionalattr" : "test" }  

Node removal

Good points:  requires single operation to remove the node from tree 

db.categoriesPCO.remove({_id:'LG'});  

Getting node children, ordered

Good points: all childs can be retrieved from database and ordered using single call. 

db.categoriesPCO.find({$query:{parent:'Electronics'}, $orderby:{order:1}})
//{ "_id" : "Cameras_and_Photography", "parent" : "Electronics", "order" : 10 }
//{ "_id" : "Shop_Top_Products", "parent" : "Electronics", "order" : 20 }
//{ "_id" : "Cell_Phones_and_Accessories", "parent" : "Electronics", "order" : 30 }

Getting all node descendants

Bad points: unfortunately, requires recursive calls to database. 

var descendants=[]
var stack=[];
var item = db.categoriesPCO.findOne({_id:"Cell_Phones_and_Accessories"});
stack.push(item);
while (stack.length>0){
    var currentnode = stack.pop();
    var children = db.categoriesPCO.find({parent:currentnode._id});
    while(true === children.hasNext()) {
        var child = children.next();
        descendants.push(child._id);
        stack.push(child);
    }
}


descendants.join(",")
//Cell_Phones_and_Smartphones,Headsets,Batteries,Cables_And_Adapters,Nokia,Samsung,Apple,HTC,Vyacheslav

Getting path to node

Bad points: unfortunately also require recursive operations to get the path. 

var path=[]
var item = db.categoriesPCO.findOne({_id:"Nokia"})
while (item.parent !== null) {
    item=db.categoriesPCO.findOne({_id:item.parent});
    path.push(item._id);
}

path.reverse().join(' / ');
//Electronics / Cell_Phones_and_Accessories / Cell_Phones_and_Smartphones 

Indexes 

Recommended index is on fields parent and order

db.categoriesPCO.ensureIndex( { parent: 1, order:1 } )  

Tree structure with childs reference 

For each node we store (ID, ChildReferences).  

 

Please note, that in this case we do not need order field, because Childs collection already provides this information. Most of languages respect the array order. If this is not in case for your language, you might consider additional coding to preserve order, however this will make things more complicated 

Adding new node

Note: requires one insert operation and one update operation to insert the node. 

db.categoriesCRO.insert({_id:'LG', childs:[]});
db.categoriesCRO.update({_id:'Electronics'},{  $addToSet:{childs:'LG'}});
//{ "_id" : "Electronics", "childs" : [     "Cameras_and_Photography", "Shop_Top_Products", "Cell_Phones_and_Accessories", "LG" ] } 

Updating/moving the node

Requires single update operation to change node order within same parent, requires two update operations, if node is moved under another parent. 

rearranging order under the same parent 

db.categoriesCRO.update({_id:'Electronics'},{$set:{"childs.1":'LG',"childs.3":'Shop_Top_Products'}});
//{ "_id" : "Electronics", "childs" : [     "Cameras_and_Photography",     "LG",     "Cell_Phones_and_Accessories",     "Shop_Top_Products" ] } 

moving the node 

db.categoriesCRO.update({_id:'Cell_Phones_and_Smartphones'},{  $addToSet:{childs:'LG'}});
db.categoriesCRO.update({_id:'Electronics'},{$pull:{childs:'LG'}});
//{ "_id" : "Cell_Phones_and_Smartphones", "childs" : [ "Nokia", "Samsung", "Apple", "HTC", "Vyacheslav", "LG" ] } 

Node removal

Node removal also requires two operations: one update and one remove. 

db.categoriesCRO.update({_id:'Cell_Phones_and_Smartphones'},{$pull:{childs:'LG'}})
db.categoriesCRO.remove({_id:'LG'}); 

Getting node children, ordered 

Bad points: requires additional client side sorting by parent array sequence. Depending on result set, it may affect speed of your code. 

var parent = db.categoriesCRO.findOne({_id:'Electronics'})
db.categoriesCRO.find({_id:{$in:parent.childs}}) 

Result set:

 { "_id" : "Cameras_and_Photography", "childs" : [     "Digital_Cameras",     "Camcorders",     "Lenses_and_Filters",     "Tripods_and_supports",     "Lighting_and_studio" ] }
{ "_id" : "Cell_Phones_and_Accessories", "childs" : [     "Cell_Phones_and_Smartphones",     "Headsets",     "Batteries",     "Cables_And_Adapters" ] }
{ "_id" : "Shop_Top_Products", "childs" : [ "IPad", "IPhone", "IPod", "Blackberry" ] }

//parent:
{
    "_id" : "Electronics",
    "childs" : [
        "Cameras_and_Photography",
        "Cell_Phones_and_Accessories",
        "Shop_Top_Products"
    ]
} 

As you see, we have ordered array childs, which can be used to sort the result set on a client

Getting all node descendants

Note: also recursive operations, but we need less selects to databases comparing to previous approach 

var descendants=[]
var stack=[];
var item = db.categoriesCRO.findOne({_id:"Cell_Phones_and_Accessories"});
stack.push(item);
while (stack.length>0){
    var currentnode = stack.pop();
    var children = db.categoriesCRO.find({_id:{$in:currentnode.childs}});

    while(true === children.hasNext()) {
        var child = children.next();
        descendants.push(child._id);
        if(child.childs.length>0){
          stack.push(child);
        }
    }
}

//Batteries,Cables_And_Adapters,Cell_Phones_and_Smartphones,Headsets,Apple,HTC,Nokia,Samsung
descendants.join(",") 

Getting path to node

Path is calculated recursively, so we need to issue number of sequential calls to database. 

var path=[]
var item = db.categoriesCRO.findOne({_id:"Nokia"})
while ((item=db.categoriesCRO.findOne({childs:item._id}))) {
    path.push(item._id);
}

path.reverse().join(' / ');
//Electronics / Cell_Phones_and_Accessories / Cell_Phones_and_Smartphones 

Indexes

Recommended index is putting index on childs: 

db.categoriesCRO.ensureIndex( { childs: 1 } ) 

Tree structure using an Array of Ancestors 

For each node we store (ID, ParentReference, AncestorReferences)

 

Adding new node 

You need one insert operation to introduce new node, however you need to invoke select to prepare the data for insert 

var ancestorpath = db.categoriesAAO.findOne({_id:'Electronics'}).ancestors;
ancestorpath.push('Electronics')
db.categoriesAAO.insert({_id:'LG', parent:'Electronics',ancestors:ancestorpath});
//{ "_id" : "LG", "parent" : "Electronics", "ancestors" : [ "Electronics" ] } 

Updating/moving the node

moving the node requires one select and one update operation 

ancestorpath = db.categoriesAAO.findOne({_id:'Cell_Phones_and_Smartphones'}).ancestors;
ancestorpath.push('Cell_Phones_and_Smartphones')
db.categoriesAAO.update({_id:'LG'},{$set:{parent:'Cell_Phones_and_Smartphones', ancestors:ancestorpath}});
//{ "_id" : "LG", "ancestors" : [     "Electronics",     "Cell_Phones_and_Accessories",     "Cell_Phones_and_Smartphones" ], "parent" : "Cell_Phones_and_Smartphones" } 

Node removal

is done with single operation 

db.categoriesAAO.remove({_id:'LG'}); 

Getting node children, unordered

 Note unless you introduce the order field, it is impossible to get ordered list of node children. You should consider another approach if you need order. 

db.categoriesAAO.find({$query:{parent:'Electronics'}}) 

Getting all node descendants

There are two options to get all node descendants. One is classic through recursion: 

var ancestors = db.categoriesAAO.find({ancestors:"Cell_Phones_and_Accessories"},{_id:1});
while(true === ancestors.hasNext()) {
       var elem = ancestors.next();
       descendants.push(elem._id);
   }
descendants.join(",")
//Cell_Phones_and_Smartphones,Headsets,Batteries,Cables_And_Adapters,Nokia,Samsung,Apple,HTC,Vyacheslav

second is using aggregation framework introduced in MongoDB 2.2:

 var aggrancestors = db.categoriesAAO.aggregate([
    {$match:{ancestors:"Cell_Phones_and_Accessories"}},
    {$project:{_id:1}},
    {$group:{_id:{},ancestors:{$addToSet:"$_id"}}}
])

descendants = aggrancestors.result[0].ancestors
descendants.join(",")
//Vyacheslav,HTC,Samsung,Cables_And_Adapters,Batteries,Headsets,Apple,Nokia,Cell_Phones_and_Smartphones 

Getting path to node

This operation is done with single call to database, which is advantage of this approach. 

var path=[]
var item = db.categoriesAAO.findOne({_id:"Nokia"})
item
path=item.ancestors;
path.join(' / ');
//Electronics / Cell_Phones_and_Accessories / Cell_Phones_and_Smartphones 

Indexes

Recommended index is putting index on ancestors 

db.categoriesAAO.ensureIndex( { ancestors: 1 } ) 

Code in action 

Code can be downloaded from repository  https://github.com/Voronenko/Storing_TreeView_Structures_WithMongoDB 

All files are packaged according to the following naming convention:

  • MODELReference.js - initialization file with tree data for MODEL approach
  • MODELReference_operating.js - add/update/move/remove/get children examples
  • MODELReference_pathtonode.js - code illustrating how to obtain path to node
  • MODELReference_nodedescendants.js - code illustrating how to retrieve all the descendants of the node 

All files are ready to use in mongo shell. You can run examples by invoking mongo < file_to_execute, or, if you want, interactively in the shell or with RockMongo web shell.  

Points of Interest 

 Please note, that MongoDB does not provide ACID transactions. This means, that for update operations split into separate update commands, your application should implement additional code to support your code specific transactions.  

Formal advise from 10gen is following:

  •  The Parent Reference pattern provides a simple solution to tree storage, but requires multiple queries to retrieve subtrees     
  •  The Child References pattern provides a suitable solution to tree storage as long as no operations on subtrees are necessary. This pattern may also provide a suitable solution for storing graphs where a node may have multiple parents.
  •  The Array of Ancestors pattern  - no specific advantages unless you constantly need to get path to the node 

You are free to mix patterns (by introducing order field, etc) to match the data operations required to your application. 

History 

I have prepared next set of examples for approaches on storing tree structures in MongoDB.

You might want to check this article, describing nested sets, Materialized path and combined approach.  

License

This article, along with any associated source code and files, is licensed under The Creative Commons Attribution-Share Alike 3.0 Unported License

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About the Author

Vyacheslav Voronenko
Web Developer
Ukraine Ukraine
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